This disclosure relates to steer-by-wire vehicle control systems.
Steering equipment for assisting a driver to steer an automobile is well known in the art. In conventional steering assemblies, the operator controls the direction of the vehicle with the aid of a steering wheel. This wheel is mechanically connected, usually through a gear assembly to the road wheels. To aid the operator, many systems utilize an auxiliary system to generate a force that is transmitted to a steering gear assembly. The additional force reduces the effort required by the operator in changing the direction of the vehicle. Typically, this auxiliary force is generated by either a hydraulic drive or an electric motor.
Because the steering wheel is connected directly to the road wheels, the resulting mechanical assembly that provides the connection can be quite complicated and expensive to produce. One advantage of having a direct connection is that the operator receives tactile feedback through the steering wheel. For example, if the vehicle changes directions while it is moving, the operator will feel resistance in the steering wheel.
In contrast, in typical steer-by-wire control systems, the steering wheel does not have a direct mechanical connection to the road wheels. Rather, the steering wheel is used to produce control signals that are transmitted to electric drive motors and actuators that are used to turn the road wheels. Typically, the angular rotational displacement of a shaft connected to the steering wheel is detected by sensors, such as position and torque sensors, and converted into an electric signal or plurality of signals. This electronic signal is processed and applied to an electric motor that is connected to and controls the angular positioning of the road wheels of the vehicle with respect to the position of the vehicle body. The road wheels are then moved according to the signal received by the motor to effect the steering of the vehicle. Because steer-by-wire steering control systems do not have a direct mechanical connection to the steered wheels, the vehicle operator does not inherently receive the tactile feedback that is provided by conventional systems. Therefore, it is desirable to incorporate mechanisms into steer-by-wire steering control systems that are adapted to provide input to a vehicle operator that simulate some of the tactile feedback characteristics of a conventional steering system.
Steer-by-wire systems commonly employ a torque feedback system to provide the operator with the appropriate tactile feedback. Commonly, the road forces are measured, compensated as necessary, and applied as a reference command to a steering wheel control system. The steering wheel control system in turn, maintains the torque at the steering wheel. In steer-by-wire systems, two methods have been considered for converting the sensed road forces back to the driver. The first method is direct, and simply commands the sensed forces to the steering wheel actuator to provide the road resistance to the driver. The advantage of this method is that the driver (almost) instantaneously feels the road forces. Although some modifications to the road force sensor signal are done, like vehicle speed modulation, there is no effort tracking in this method. As such, the modulated signal is commanded in a direct (or open loop) fashion to the steering wheel actuator. With careful tuning, this method is adequate. But, due to lack of feedback, the resulting steering feel to the driver will be sensitive to changes in the system or component variations.
The second method once again utilizes the (modulated) road force sensor, but then compares it in a feedback loop with a torque sensor signal to generate an error signal. Following appropriate compensation, a final command is given to the steering wheel actuator. The advantage of this method is its insensitivity to variations (and or disturbances) in the steering wheel system. Unfortunately, to ensure adequate tactile feedback of road forces, the loop gain generally needs to be quite high, thereby making the system potentially prone to instability, noise susceptibility, and the like.
Therefore, it is considered advantageous to provide a steer-by-wire steering control system that provides desirable immediate tactile feedback to the operator and is less sensitive to induced disturbances, but avoids instability and noise susceptibility.
A steer-by-wire control system comprising a master control unit, a road wheel unit electrically connected to the master control unit and a steering wheel unit electrically connected to the master control unit. The steering wheel unit includes a feed forward control path and a feed back control path.
A method of steering a vehicle comprising producing a tie rod force signal, a road wheel position signal, a steering wheel position signal, a steering wheel torque signal, and a vehicle speed signal. The method also includes generating a steering wheel torque command signal, a road wheel command signal and controlling a steering wheel unit with a feed forward control path and a feed back control path.
A storage medium encoded with a machine-readable program code for steering a vehicle, the storage medium including instructions for causing a computer to implement a method comprising producing a tie rod force signal, a road wheel position signal, a steering wheel position signal, a steering wheel torque signal, and a vehicle speed signal. The method also includes generating a steering wheel torque command signal, a road wheel command signal and controlling a steering wheel unit with a feed forward control path and a feed back control path.
A computer data signal for steering a vehicle, the computer data signal comprising code configured to cause a computer to implement a method comprising producing a tie rod force signal, a road wheel position signal, a steering wheel position signal, a steering wheel torque signal, and a vehicle speed signal. The method also includes generating a steering wheel torque command signal, a road wheel command signal and controlling a steering wheel unit with a feed forward control path and a feed back control path.